Devices for heat transfer

ABSTRACT

Devices and methods of heat transfer are provided. In an example, a thermo-reversible hydrogel is provided in a first portion of a casing of a device. A wicking surface is provided in an inner surface of the casing between the first portion and the second portion.

BACKGROUND

Heat transfer devices are used to transfer heat between a heat sourceand a heat sink. Heat transfer devices include two regions, a firstregion coupled to the heat source and a second region coupled to theheat sink. In the first region, heat is received from the heat sourceand is then transferred to the second region, for example, byconduction, convection, radiation, phase transition, and the like.Subsequently, heat is transferred from the second region to the heatsink.

BRIEF DESCRIPTION OF DRAWINGS

The following detailed description references the figures, wherein:

FIG. 1 illustrates an example device for heat transfer, according to anexample implementation of the present subject matter;

FIG. 2 is an example method of heat transfer, according to an exampleimplementation of the present subject matter;

FIG. 3(a) illustrates an example heat pipe, according to an exampleimplementation of the present subject matter;

FIG. 3(b) illustrates an example heat pipe, according to another exampleimplementation of the present subject matter,

FIG. 4 illustrates an example vapor chamber, according to an exampleimplementation of the present subject matter;

FIG. 5 illustrates an example device for heat transfer, according to anexample implementation of the present subject matter; and

FIG. 6 illustrates an example method of preparing a device for heattransfer, according to an example implementation of the present subjectmatter.

DETAILED DESCRIPTION

Heat transfer devices include devices such as heat pipes and vaporchambers. Heat transfer devices can be used in various systems, such asin spacecraft, electronic devices, solar heat transfer systems, and thelike. Generally, heat transfer devices that work on principles of phasetransition include a sealed casing enclosing a working fluid of highheat capacity. The working fluid is selected based on compatibility withthe casing. For example, when the casing is made of copper, the workingfluid can be water.

In such a heat transfer device, during operation, working fluidevaporates, for example, in an evaporation area dose to a heat source.The vapors are transferred to a second region where the vapors condense,for example, in a condensation area close to a heat sink. To return thecondensed working fluid to the evaporation area for subsequent heattransfer, a fluid transfer mechanism, such as capillary action of awicking surface, may be used.

The present subject matter relates to devices for heat transfer withincreased heat dissipation performance, and methods of preparing heattransfer devices. An example device for heat transfer includes a casing.The casing includes a first portion to receive heat from a heat source.A thermo-reversible hydrogel is provided in contact with an innersurface of the first portion and is soaked in a working fluid. A wickingsurface is also provided along the inner surface of the casing. Thecasing further includes a second portion, which is disposedsubstantially opposite to the first portion. The first portion andsecond portion are fluidly coupled by the wicking surface and a vaporregion.

The device of the present subject matter can be prepared by sintering awicking material, such as copper powder, on an inner surface of thecasing followed by coating the thermo-reversible hydrogel in the firstportion of the casing and drying the hydrogel.

In operation, in a first example, when a first temperature of the firstportion is higher than a second temperature of the second portion,vapors of the working fluid are formed at the first portion. This casearises when, for example, the heat source that is in contact with thefirst portion is switched on. The vapors of the working fluid aretransferred to the second portion through the vapor region. At thesecond portion, the vapors are condensed and the condensed working fluidis transferred to the first portion by the wicking surface. At the firstportion, the condensed working fluid is absorbed by the hydrogel,thereby increasing the rate of return of the condensed working fluid andincreasing heat dissipation.

On the other hand, in a second example, when the second temperature ishigher than the first temperature, vapors of the working fluid areformed at the second portion. This case arises when, for example, theheat source is switched off and the first portion cools down faster thanthe second portion. In this case, the vapors of the working fluid aretransferred from the second portion to the first portion, forcondensation, through the vapor region, while the condensed workingfluid is transferred from the first portion to the second portion by thewicking structure. Further, at the first portion, the vapors areabsorbed by the hydrogel, thereby increasing a rate of return of vaporsand increasing heat dissipation.

The thermo-reversible hydrogel in the device increases rate ofdissipation of heat from the working fluid by absorption of thecondensed working fluid in the first example and absorption of thevapors in the second example. Therefore, the device of the presentsubject matter can be used for rapid cooling.

The present subject matter provides devices for heat transfer whichprovide rapid cooling without substantial increase in weight. Therefore,the devices can be used to cool devices, such as Liquid Crystal Display(LCD) panels, Light Emitting Diodes (LEDs), Central Processing Units(CPUs) and the like. Further, the increased heat dissipation performanceand rapid cooling helps in increasing power efficiency and reducingrisks of explosion due to overheating.

The following description refers to the accompanying drawings. Whereverpossible, the same reference numbers are used in the drawings and thefollowing description to refer to the same or similar parts. Whileseveral examples are described in the description, modifications,adaptations, and other implementations are possible. Accordingly, thefollowing detailed description does not limit the disclosed examples.Instead, the proper scope of the disclosed examples may be defined bythe appended claims.

Example implementations of the present subject matter are described withregard to personal computers (PCs) and laptop computers. Although notdescribed, it will be understood that the implementations of the presentsubject matter can be used with other types of devices as well, such astelevisions, tablets, smartphone devices, solar panels, aircraft and thelike.

FIG. 1 illustrates an example device 100 for heat transfer, according toan example implementation of the present subject matter. The device 100may be used, for example, in electronic circuitry, spacecraft, and thelike. Based on the application, the device 100 may be fabricated as amicro-device or a nano-device.

In an example, the device 100 may be one of a heat pipe and a vaporchamber. A heat pipe is a substantially hollow, cylindrical heattransfer device provided between a heat source and a heat sink totransfer heat between the heat source and a heat sink. A vapor chambermay be understood as a flattened heat pipe with substantially planarheat transfer surfaces and a hollow region therebetween. Function ofheat pipes and vapor chambers is based on principles of conduction andphase transition.

The device 100 includes a casing 102. In an example, the casing 102 isfabricated from a material which has high thermal conductivity. Forexample, the casing may be fabricated from a metal, such as copper,aluminum, alloy, and the like. In an example, when the device 100 is aheat pipe, the casing 102 can be fabricated as an elongated hollowcylinder with both ends of the cylinder sealed. In another example, whenthe device 100 is a vapor chamber, the casing 102 can be fabricated as ahollow, longitudinally flattened structure.

The casing 102 includes a first portion 104. In an example, when thedevice 100 is a heat pipe, the first portion 104 is presentsubstantially towards a first end of the heat pipe as will be explainedlater with reference to FIGS. 3(a) and 3(b). In an example, when thedevice 100 is a vapor chamber, the first portion 104 is presentsubstantially towards a first flattened surface of the vapor chamber aswill be explained later with reference to FIG. 4. The first portion 104is to receive heat from a heat source 106. The first portion 104 may,therefore, be coupled to the heat source 106.

The heat source 106 is a device from which heat is to be removed, forexample, for cooling the device. For this, heat from the heat source 106is received by the device 100. For example, in an electronic device,such as a computing device, the heat source 106 may be a CentralProcessing Unit (CPU) which may get heated during start-up and runningof the computing device and hence may have to be cooled by using thedevice 100. The casing 102 of the device 100 receives heat from the heatsource 106 at the first portion 104.

The casing 102 also includes a working fluid (not shown). The workingfluid is selected based on thermal conductivity and compatibility withthe material of the casing 102. For example, when the casing 102 is acopper casing, the working fluid can be water, ethanol, and the like.

A thermo-reversible hydrogel 108 is provided in the first portion 104and is soaked in the working fluid. The thermo-reversible hydrogel 108may be coated in the first portion 104. In an example, a thickness ofthe thermo-reversible hydrogel 108 in the first portion 104 is in arange of 100-800 μm. Thermo-reversible hydrogels are hydrogels whichform a gel when cooled and form a viscous fluid state when exposed toheat. Thermo-reversible hydrogels, therefore, do not undergo permanentchange. Further, transition from gel to viscous fluid and vice versa canbe performed repeatedly based on heat received by the hydrogel. Thethermo-reversible hydrogel 108 may be made of polymers selected fromethylene maleic anhydride copolymer, carboxymethyl cellulose, polyvinylalcohol copolymers, starch grafted copolymer of polyacrylonitrile orpolyacrylamide super absorbents, and combinations thereof.

In operation, when no heat is received, the thermo-reversible hydrogel108 retains its gel form. In the gel form, molecules of thethermo-reversible hydrogel 108 form a three-dimensional cross-linkednetwork where the network traps the working fluid. When thethermo-reversible hydrogel 108 receives heat from the heat source 106through casing 102, the gel forms a viscous fluid, thereby releasing theworking fluid for phase transition and heat transfer.

The working fluid evaporates due to the heat and forms vapors which aretransferred, for example, by diffusion to a second portion 110 of thecasing 102 of the device 100. The second portion 110 is presentsubstantially opposite to the first portion 104. In an example, when thedevice 100 is the heat pipe, the second portion 110 is the second end ofthe cylinder as will be explained later with reference to FIGS. 3(a) and3(b). In an example, when the device 100 is the vapor chamber, thesecond portion 110 is the second flattened surface of the vapor chamberas will be explained later with reference to FIG. 4.

In the present description, a temperature of the first portion 104 isreferred to as a first temperature and a temperature of the secondportion 110 is referred to as a second temperature.

At the second portion 110, as the second temperature of the secondportion 110 is lower than the first temperature of the first portion104, the vapors condense. The condensed vapors are transferred to thefirst portion 104 by a wicking surface 112 by capillary forces. Thewicking surface 112 is provided along an inner surface of the casing 102between the first portion 104 and the second portion 110. In an example,the wicking surface 112 may extend into the first portion 104 and thesecond portion 110. The wicking surface 112 may be one of a sinteredmetal powder, a screen, and a grooved wicking surface. In an example,the wicking surface 112 is fabricated from the material of the casing102. For example, when the casing 102 is copper, the wicking surface 112may be formed from sintered copper powder.

FIG. 2 depicts an example method of heat transfer in device 100,according to an example implementation of the present subject matter. Atblock 202, heat is received at the first portion 104 of the casing 102.On receiving heat, the thermo-reversible hydrogel 108 releases theworking fluid. The heat causes the working fluid to form vapors. Thevapors diffuse towards the second portion 110. At the second portion110, at block 204, the vapors of the working fluid are cooled. Thevapors, therefore, condense at the second portion 110. The wickingsurface 112 transfers the condensed working fluid from the secondportion 110 to the first portion 104 for absorption by thethermo-reversible hydrogel 108. The method of heat transfer is furtherexplained with respect to FIGS. 3(a) and 4.

In an example, the second temperature may be higher than the firstportion, for example, when the first portion 104 loses heat faster thanthe second portion 110. In this example, vaporization of the workingfluid is caused at the second portion 110. The vapors then diffuse fromthe second portion 110 to the first portion 104 where the vapors arecondensed and absorbed by the thermo-reversible hydrogel 108. Thisexample is further explained with respect to FIG. 3(b).

FIG. 3(a) depicts operation of an example heat pipe 300 when heat isreceived from a heat source, according to an example implementation ofthe present subject matter. Hereinafter, ends of the heat pipe 300 arereferred to as bases. The heat pipe 300 includes the casing 102 which issubstantially cylindrical and includes a first base 302 and a secondbase 304. The first portion 104 of the casing 102 of the heat pipe 300is present substantially towards the first base 302 and the secondportion 110 of the casing 102 of the heat pipe 300 is presentsubstantially towards the second base 304. In an example, the first base302 may be rounded to increase surface area available for coating thethermo-reversible hydrogel 108.

A working fluid 306 is provided in the first portion 104. Thethermo-reversible hydrogel 108 is soaked in the working fluid 306. In anexample, when the thermo-reversible hydrogel 108 is saturated with theworking fluid 306, any excess working fluid 306 may be retained unboundin the first portion 104. The first portion 104 may receive heat from aheat source (not shown).

When heat is supplied to the first portion 104 as depicted by arrows308, any excess working fluid 306 vaporizes and additionally, thethermo-reversible hydrogel 108 releases the soaked working fluid 306 forvaporization. The vapors 310 are transferred to the second portion 110,for example, by diffusion. As the second temperature is less than thefirst temperature, the vapors 310 are condensed. In an example, thesecond portion 110 may be open to surrounding environment at ambientconditions, and, therefore, may be at a lower temperature than the firstportion 104. During condensation, the vapors 310 reject heat at thesecond portion 110 as shown by arrows 312. The condensed working fluid314 is then transferred by capillary action by the wicking surface 112.

The operation as depicted in FIG. 3(a) may occur during start-up andrunning of the computing device. For example, during start-up, aprocessing unit of a computing device may generate heat and act as theheat source. Therefore, the heat pipe 300 may be disposed such that thefirst portion 104 is in close proximity to the processing unit. Forexample, the heat pipe 300 may be coupled to an enclosure housing theprocessing unit. The second portion 110 may be disposed such that thesecond portion 110 is in dose proximity to other components on thecomputing device. In an example, the second portion 110 may be open toambient conditions. In another example, the second portion 110 may bedisposed in proximity to a fan to increase heat rejection at the secondportion 110.

However, when the computing device is switched off, the processing unitmay cool down to a temperature lower than that in the second portion,i.e., the second temperature may be higher than the first temperature.In another example, the second portion may be at a higher temperature,for example, due to the ambient temperature bring higher than thetemperature in the first portion. This example is as depicted in FIG.3(b).

FIG. 3(b) depicts operation of the example heat pipe 300 when the secondtemperature is higher than the first temperature, according to anexample implementation of the present subject matter. As shown in FIG.3(b), the heat pipe 300 receives heat at the second portion 110 as shownby arrows 308. The received heat causes vaporization of working fluid atthe second portion 110. The vapors 310 of the working fluid aretransferred from the second portion 110 to the first portion 104 wherethe vapors 310 are condensed and absorbed by the hydrogel. The condensedworking fluid 314 is then transferred from the first portion 104 to thesecond portion 110 by the wicking surface 112. This facilitates furthercycles of heat transfer as more of the previously vaporized workingfluid returns to the hydrogel.

In another example, the device for heat transfer may be a vapor chamber.FIG. 4 depicts operation of an example vapor chamber 400, according toan example implementation of the present subject matter. The vaporchamber 400 includes the casing 102 which includes a first flattenedsurface 402 and a second flattened surface 404 which are substantiallyopposite to each other. The first portion 104 is present substantiallyproximate to the first flattened surface 402 and the second portion 110is present substantially proximate to the second flattened surface 404.

In one example, the first flattened surface 402 includes well structures406 for holding the thermo-reversible hydrogel 108. However, thethermo-reversible hydrogel 108 may be provided on the first flattenedsurface 402 without the well structures 406. In another example, thethermo-reversible hydrogel 108 may be coated over the wicking surface112 in the first portion 104.

In operation, when heat is supplied at the first portion 104 as shown byarrows 406, the thermo-reversible hydrogel 108 releases absorbed workingfluid which vaporizes. The heat may be supplied to the first portion104, for example, by a heat source (not shown). The vapors as depictedby arrows 408 diffuse within the vapor chamber 400. Since the secondtemperature of the second portion is lower than the first temperature ofthe first portion, the vapors 408 condense. In an example, the secondportion 110 may be coupled to a condenser to condense the vapors 408.The condensed vapors 410 are then transferred to the thermo-reversiblehydrogel 108 by the wicking surface 112 provided between the firstportion 104 and the second portion 110. It will be understood that thevapor chamber 400 will also facilitate in removing heat from the secondportion 110 and return of the vaporized working fluid back to thehydrogel when the first portion cools to a temperature lower than thesecond portion, as discussed above with reference to the heat pipe 300.

FIG. 5 illustrates another example device 500 for heat transfer,according to an example implementation of the present subject matter.The device 500 may comprise a casing (not shown). The device 500includes a first portion 502 and a second portion 504. In an example,the casing may comprise the first portion 502 and the second portion504. The first portion 502 includes a thermo-reversible hydrogel 506soaked in a working fluid 508. The thermo-reversible hydrogel 506 maybe, for example, coated on a surface of the first portion 502. In anexample, multiple layers of the thermo-reversible hydrogel 506 may beprovided in the first portion 502. Each layer may be a same hydrogel ora different hydrogel. In an example, the thermo-reversible hydrogel 506can be provided as patterns or grooves to increase surface area ofabsorption and, thereby, heat dissipation.

A vapor region 510 is provided in between and fluidly couples the firstportion 502 and the second portion 504. The vapor region 510 is enclosedby a wicking surface 512. The wicking surface 512 also fluidly couplesthe first portion 502 and the second portion 504. The vapor region 510and the wicking surface 512 are for heat transfer between the firstportion 502 and the second portion 504.

In operation, for example, during start-up of an electronic device, heatis received at the first portion 502, for example, due to contact with aheat source, such as a processor. When heat is received at the firstportion 502, a first temperature of the first portion 502 becomes higherthan a second temperature of the second portion 504. In this example,the vapor region 510 is to transfer vapors of the working fluid 508 fromthe first portion 502 to the second portion 504 for condensation and thewicking surface 512 is to transfer condensed working fluid from thesecond portion 504 to the first portion 502. At the first portion 502,the thermo-reversible hydrogel 506 is to absorb the condensed workingfluid.

Further, for example, when the electronic device is switched off or whenan ambient temperature is high, the second temperature of the secondportion 504 may be higher than a first temperature of the first portion502. Therefore, at the second portion 504, working fluid vaporizes. Inthis example, the vapor region 510 is to transfer vapors of the workingfluid 508 from the second portion 504 to the first portion 502 forcondensation and the wicking surface 512 is to transfer condensedworking fluid from the first portion 502 to the second portion 504. Thethermo-reversible hydrogel 506 is to absorb the vapors at the firstportion 502.

As explained, the thermo-reversible hydrogel 506 increases rate ofdissipation of heat from the working fluid 508 by absorption of thecondensed working fluid and absorption of the vapors. Therefore, theheat transfer device of the present subject matter can be used for rapidcooling without substantially increasing weight of the heat transferdevices.

FIG. 6 depicts a method 600 of preparing a heat transfer device,according to an example implementation of the present subject matter.The order in which the method 600 is described is not intended to beconstrued as a limitation, and any number of the described method blocksmay be combined in any order to implement the method 600, or alternativemethods. Although the method 600 may be implemented in a variety ofsystems, the method 600 is explained in relation to the aforementioneddevices for heat transfer, for ease of explanation.

At block 602, a wick material powder is sintered along an inner surfaceof a casing of a device for heat transfer. As will be understood,sintering is a process of forming a solid, porous mass of the wickmaterial without melting the wick material powder to liquefaction. In anexample, prior to sintering of the wick material powder, a first portionof the casing may be sealed, for example, by welding. For sintering, thewick material powder is filled into the casing. In an example, thesintering is performed at about 700-1300° C. for about 30-60 minutes.Sintering of the wick material powder helps in forming a wicking surfacealong an inner surface of the casing. In an example, the device for heattransfer may be device 100, 300, 400 and the casing may be casing 102,respectively. In another example, the device may be device 500 which mayinclude a casing (not shown).

After sintering, at block 604, a thermo-reversible hydrogel is coated ata first portion of the casing. In an example, the thermo-reversiblehydrogel is coated on top of the wicking surface. In another example,the wicking surface may be scrapped or removed by other techniques andthe thermo-reversible hydrogel may be coated directly on the casing. Inan example, the first portion may be first portion 104 of the device100, 300, 400. In another example, first portion may be first portion504 of device 500.

The thermo-reversible hydrogel is coated by spraying a solutioncomprising the thermo-reversible hydrogel on the inner surface of thecasing at the first portion. In an example, the solution includes thethermo-reversible hydrogel in a range of about 0.1-3.0% weight by volumeof the solution. The solution may be made with a working fluid as asolvent. The thermo-reversible hydrogel may be sprayed at a pressure ina range of about 0.0005-0.002 Torr. In an example, multiple layers ofthe thermo-reversible hydrogel may be coated at the first portion.

After coating the thermo-reversible hydrogel, at block 606, thethermo-reversible hydrogel is dried. In an example, thethermo-reversible hydrogel is dried at a temperature of about 105-120°C. for about 15-40 minutes.

The drying of the thermo-reversible hydrogel is followed by injecting aworking fluid into the first portion of the casing under vacuum. Thishelps increase the amount of working fluid absorbed by the hydrogel. Theamount of working fluid injected may be varied so that the hydrogel issoaked in the working fluid and some excess working fluid remains incontact with the hydrogel.

Therefore, the methods and devices of the present subject matter providean increase in rate of heat dissipation without increasing weight of thedevice. The increased rate of heat dissipation is due to absorption ofcondensed working fluid and absorption of vapor by the thermo-reversiblehydrogel. Increased heat dissipation reduces chances of damage of theheat source, for example, due to overheating.

The preceding description has been presented to illustrate and describeexamples of the principles described. This description is not intendedto be exhaustive. Many modifications and variations are possible inlight of the above teaching.

We claim:
 1. A device for heat transfer comprising: a casing, whereinthe casing comprises: a first portion to receive heat from a heatsource; a thermo-reversible hydrogel provided in the first portion; anda wicking surface provided along an inner surface of the casing andconnecting the first portion to a second portion of the casing.
 2. Thedevice of claim 1, wherein the device is one of a heat pipe and a vaporchamber.
 3. The device of claim 1, wherein the thermo-reversiblehydrogel is selected from ethylene maleic anhydride copolymer,carboxymethyl cellulose, polyvinyl alcohol copolymers, starch graftedcopolymer of polyacrylonitrile or polyacrylamide super absorbents, andcombinations thereof.
 4. The device of claim 1, wherein the wickingsurface is one of a sintered metal powder, a screen, and a groovedwicking surface.
 5. The device of claim 1, wherein a thickness of thethermo-reversible hydrogel is in a range of 100-800 μm.
 6. The device ofclaim 1, wherein the first portion comprises a working fluid and whereinthe thermo-reversible hydrogel is soaked in the working fluid.
 7. Thedevice of claim 6, wherein the casing and the wicking surface arefabricated from copper and wherein the working fluid is water.
 8. Amethod of preparing a device for heat transfer, the method comprising:sintering a wick material powder along an inner surface of a casing ofthe heat transfer device; coating a thermo-reversible hydrogel on aninner surface of a first portion of the casing of the heat transferdevice; and drying the thermo-reversible hydrogel.
 9. The method ofclaim 8, wherein coating the thermo-reversible hydrogel comprisesspraying a solution comprising the thermo-reversible hydrogel on theinner surface at a pressure in a range of about 0.0005-0.002 Torr,wherein the solution comprises the thermo-reversible hydrogel in a rangeof about 0.1-3.0% weight by volume of the solution.
 10. The method ofclaim 8, wherein sintering is performed at about 700-1300° C. for about30-60 minutes.
 11. The method of claim 8, wherein drying of thethermo-reversible hydrogel is performed at a temperature of about105-120° C. for about 15-40 minutes.
 12. The method of claim 8, whereinthe method further comprises injecting a working fluid into the firstportion of the casing under vacuum.
 13. A device comprising: a firstportion comprising a thermo-reversible hydrogel soaked in a workingfluid; a second portion substantially opposite to the first portion; avapor region between the first portion and the second portion; and awicking surface enclosing the vapor region, wherein the vapor region andthe wicking surface fluidly couple the first portion and the secondportion for heat transfer between the first portion and the secondportion.
 14. The device as claimed in claim 13, wherein, when a firsttemperature of the first portion is higher than a second temperature ofthe second portion: the vapor region is to transfer vapors of theworking fluid from the first portion to the second portion forcondensation; the wicking surface is to transfer condensed working fluidfrom the second portion to the first portion; and the thermo-reversiblehydrogel is to absorb the condensed working fluid.
 15. The device asclaimed in claim 13, wherein, when a second temperature of the secondportion is higher than a first temperature of the first portion: thevapor region is to transfer vapors of the working fluid from the secondportion to the first portion for condensation; the wicking surface is totransfer condensed working fluid from the first portion to the secondportion; and the thermos-reversible hydrogel is to absorb the vapors.